Used to create, update, delete and browse through outgoing SQL notifications.

A notification is an SQL query invoked periodically which upon its execution
runs asynchronously a user-defined service providing the service with all the
rows fetched by the query, if any, under a ‘data’ key of the input dictionary
in self.request.raw_request. The service will be invoked with empty input
if the database doesn’t return anything in a given iteration.